The requirements and benefits of cooling the surface of a duct carrying a high temperature fluid, such as the exhaust duct of a gas turbine engine, are well known and have long been the subject of technical development. One effective method for achieving such interior duct surface cooling is to establish and maintain a layer of relatively cool fluid adjacent the surface to prevent contact with the conducted heated fluid.
For continuous protection around the entire duct perimeter and along the entire duct length, it has been found advantageous to use a louver type system for introducing the layer of relatively cool fluid adjacent the duct surface. The louvers are oriented substantially transverse with respect to the heated fluid flow, circumscribing the duct perimeter and being disposed at one or more axial distances along the duct length.
U.S. Pat. No. 3,925,982 issued to Mueller shows an axisymmetric rocket nozzle having a plurality of louver like openings disposed about the perimeter of the inner nozzle surface. Each axial location of a louver is supplied with cooling fluid by an external manifold.
The Mueller system and similar methods for maintaining the cooling layer adjacent the duct surface are well suited for round, axisymmetric duct cross sections wherein the static pressure gradient of the conducted fluid varies only in the axial flow direction. Such systems are less effective, however, in asymmetric flow configurations, such as square, rectangular, or thrust vectoring duct configurations wherein the static pressure gradient within the duct varies not only streamwise, but also between various locations about the duct perimeter.
As will be appreciated by those skilled in the art, a cooling fluid supply manifold disposed about the perimeter of a duct having a non-uniform perimeter static pressure distribution will deliver a greater volume of cooling fluid to the relatively low pressure locations at the duct surface and somewhat less volume to the relatively high pressure locations. In extreme cases of pressure imbalance about the perimeter, the ingestion of hot fluid into the louver and/or supply manifold may occur, possibly causing rapid deterioration of the duct surface and cooling supply system as a result of local overheating. Increasing the overall cooling fluid delivery rate to insure that no portion of the duct perimeter is without sufficient cooling flow results in overcooling of the relatively low pressure locations within the duct.
What is needed is a cooling system able to establish and maintain a uniform layer of cooling fluid adjacent the surface of a duct having a non-uniform surface static pressure distribution about the duct perimeter.